Attorney-General Mark Dreyfus has written an open letter to Tony Abbott, calling on the Opposition Leader back away from a pledge to repeal a section of the Racial Discrimination Act.

Mr Dreyfus wrote to Mr Abbott on Monday morning, arguing the Coalition's stance on section 18C of the Racial Discrimination Act is inconsistent with its support for the London Declaration on Combatting Anti-Semitism.

The London Declaration aims to draw international attention to the resurgence of anti-Semitism, and has been signed by politicians around the world, including UK Prime Minister David Cameron and Canadian Prime Minister Stephen Harper.

Prime Minister Julia Gillard signed the declaration in April and earlier this month Mr Abbott and a group of more than 40 Coalition MPs also signed up.

Mr Dreyfus argues there is now an inconsistency between Mr Abbott's support for the London Declaration, which states that parliamentarians should "legislate effective Hate Crime legislation" and his pledge to repeal section 18C of the Racial Discrimination Act, if elected.

Section 18C makes it unlawful to publish material which offends or insults a person or group because "of the race, colour or national or ethnic origin of the person or of some or all of the people in the group".

"Accordingly, section 18C is precisely the kind of legislated protection against anti-Semitism and discrimination that the London Declaration calls on its signatories to enact," Mr Dreyfus' letter states.

Last month Mr Abbott told the Institute of Public Affairs that if elected: "We will repeal Section 18C of the Racial Discrimination Act, at least in its current form."

The Coalition has had reservations about 18C, arguing it hampers free speech, since 2011 when columnist Andrew Bolt was found to have contravened the section for comments he made about a group of indigenous Australians.

Mr Dreyfus, who is the most senior Jewish MP in the Gillard government, said he was "very pleased" to see opposition members sign the London Declaration

"However [it is] apparent that they haven't quite understood what they have agreed," Mr Dreyfus told Fairfax Media on Monday.

Mr Dreyfus said that if 18C was repealed it would carry the implication that "Australia no longer cares about racial hate speech - that as a country we don't believe there should be legal prohibitions against incitement to racial hatred."

The Attorney-General also called on Mr Abbott to be specific about what he would change about 18C.

"Be specific. Don't just talk in three word slogans. What are you going to do?"